GGDEF and EAL domain proteins were identified in the chromosome of many bacteria. GGDEF domain proteins are generally diguanyl cyclases which synthesize the secondary messenger cyclic di-GMP. EAL domain proteins are generally c-di-GMP specific phosphodiesterase which degrade c-di-GMP. C-di-GMP is involved in the regulation of multiple phenotypes in bacteria like biofilm formation, motility and virulence. In this work, we found that c-di-GMP inversely regulates biofilm formation and motility in bacteria such as Salmonella enterica serovar Typhimurium, Escherichia coli and Pseudomonas aeruginosa. Thereby, GGDEF domain proteins up-regulate c-di-GMP concentrations and the biofilm phenotype, while down regulating the motility phenotype. EAL domain proteins have the opposite effect. Consequently, cyclic di-GMP controls the transition from sessility to motility. Phenotypic convergence is also observed when GGDEF domain proteins are exchanged between different species. Mutual complementation of GGDEF domain proteins mutants of S. Typhimurium and Yersinia pestis restored the respective phenotypes, cellulose biosynthesis and biofilm formation. In S. Typhimurium, the rdar morphotype, a multcellular behaviour that involves biofilm formation is regulated by c-di-GMP. The rdar morphotype is characterized by the expression of extracellular matrix components cellulose and curli fimbriae. CsgD is a major transcriptional regulator required for the activation of biosynthesis of cellulose and curli fimbriae. Systematic analysis of the 5 GGDEF, 7 EAL and 7 GGDEF-EAL domain proteins encoded on the S. Typhimurium chromosome indicates that these proteins do have overlapping, but never a redundant function. Mutant analysis revealed that the GGDEF-EAL domain proteins STM2123 and STM3388 enhance the expression of the transcriptional regulator CsgD at the transcriptional and post-transcriptional level and, consequently, rdar morphotype development, although no change in intracellular c-di-GMP levels is observed. Chromosomal inactivation of ...